Fingerprint verification device

ABSTRACT

A fingerprint verification device includes a fingerprint sensor including an imaging surface. The fingerprint sensor is configured to capture images of a user&#39;s finger when the finger is swiped against the imaging surface. A memory stores fingerprint data. A controller compares minutiae in the captured images with the stored fingerprint data to verify the user&#39;s identity. The controller is configured to determine whether the finger is a real human finger based on the spacing of minutiae in the captured images.

FIELD OF THE INVENTION

[0001] The present invention generally relates to biometric devices, andmore particularly to a fingerprint verification device.

BACKGROUND OF THE INVENTION

[0002] In recent years, fingerprints have played an important role in afield known as biometrics. Biometrics refers to the field of technologydevoted to identification of individuals using biological traits.Fingerprints may be acquired in the form of digital images that areamenable to processing by computer software. There are a number ofexisting techniques for capturing a fingerprint in a digital format,including capacitive sensors, thermal sensors, and optical sensors.

[0003] Capacitive sensors have problems with electrostatic discharge(ESD), and with sensing dry fingers. Thermal sensors use a heater andcorrespondingly have an undesirable heat up lag time. Optical sensors donot use a heater, typically have less of a problem with ESD thancapacitive sensors, and are better able to sense a dry finger. Butcurrent optical sensors have a problem with distinguishing between areal human finger and a “fake” finger, such as a photocopy of a realhuman finger.

[0004] Fingerprint sensors can be classified as either area sensors orswipe sensors. Swipe sensors are also referred to as linear sensors,since they typically use a linear array, as opposed to an area array, ofsensing elements. With an optical area sensor, images of a finger arecaptured while the finger is held stationary on the area array ofsensing elements. With an optical swipe sensor, a user slides or swipeshis finger on the sensor, which captures multiple images during theswipe. The sequence of captured images is then combined or “stitched”together to form one continuous fingerprint image.

[0005] Area sensors typically take up more space than swipe sensors. Insome applications, such as for some portable electronic devices (e.g.,cellular telephones, personal digital assistants (PDA's), laptopcomputers, etc.), it is undesirable to have a large area dedicated toplacing a finger on for the purpose of finger recognition.

SUMMARY OF THE INVENTION

[0006] One form of the present invention provides a fingerprintverification device. The device includes a fingerprint sensor includingan imaging surface. The fingerprint sensor is configured to captureimages of a user's finger when the finger is swiped against the imagingsurface. A memory stores fingerprint data. A controller comparesminutiae in the captured images with the stored fingerprint data toverify the user's identity. The controller is configured to determinewhether the finger is a real human finger based on the spacing ofminutiae in the captured images.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a simplified schematic diagram illustrating an opticalfingerprint swipe sensor according to one embodiment of the presentinvention.

[0008]FIG. 2 is a diagram illustrating an image sensor for the opticalfingerprint swipe sensor shown in FIG. 1 according to one embodiment ofthe present invention.

[0009]FIG. 3 is a block diagram illustrating major components of animage sensing and processing device for the optical fingerprint swipesensor shown in FIG. 1 according to one embodiment of the presentinvention.

[0010]FIG. 4A is a simplified diagram illustrating an examplefingerprint image captured by an optical area sensor.

[0011]FIG. 4B is a simplified diagram illustrating a stitchedfingerprint image generated from images captured during a swipe-downmotion according to one embodiment of the present invention.

[0012]FIG. 4C is a simplified diagram illustrating a stitchedfingerprint image generated from images captured during a swipe-upmotion according to one embodiment of the present invention.

[0013]FIG. 5 is a flow diagram illustrating a fingerprint enrollmentmethod according to one embodiment of the present invention.

[0014]FIG. 6 is a flow diagram illustrating a fingerprint authenticationmethod according to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] In the following detailed description of the preferredembodiments, reference is made to the accompanying drawings, which forma part hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

[0016]FIG. 1 is a simplified schematic diagram illustrating an opticalfingerprint swipe sensor 100 according to one embodiment of the presentinvention. Fingerprint swipe sensor 100 includes light source 106,finger prism 108, optics 110, sensor prism 112, and image sensing andprocessing device 114. Image sensing and processing device 114 includesimage sensor 116. In one embodiment, sensor 100 is implemented in theform of a module that may be used in portable electronic host devices,such as laptop computers, personal digital assistants (PDA's), cellulartelephones, as well as other types of devices. Fingerprint sensor 100may also be used in many other types of host devices, whether portableor not, where finger recognition functionality is desired. In one formof the invention, image sensing and processing device 114 is implementedas an integrated circuit (IC) chip. In one embodiment, image sensor 116is a complimentary metal oxide semiconductor (CMOS) image sensor. In oneform of the invention, in addition to the optics shown in FIG. 1, sensor100 also includes additional optical parts, such as more lenses and anaperture.

[0017] Finger prism 108 includes first (swipe) surface 108A, secondsurface 108B, and third surface 108C. A user swipes fingertip 104 offinger 102 across the swipe surface 108A of prism 108. In oneembodiment, the swipe surface 108A is about 9 mm long, 1 mm wide at bothends, and 2 mm wide near the center. Light from light source 106illuminates fingertip 104. Although a single light source 106 is shownin FIG. 1, in other embodiments, multiple light sources 106 are used. Inone form of the invention, light source 106 includes one or more lightemitting diodes (LED's). The light reflected from fingertip 104 isreflected by surface 108C, passes through surface 108B, and is directedby optics 110 onto sensor prism 112.

[0018] Sensor prism 112 includes first surface 112A, second surface112B, and third surface 112C. The light directed onto sensor prism 112from optics 110 passes through surface 112A, is reflected by surface112B, passes through surface 112C, and strikes image sensor 116. Device114 processes images captured by image sensor 116 as described infurther detail below.

[0019] When a user slides or swipes his finger 102 on the finger prism108, image sensor 116 captures multiple images of the finger 102 duringthe swipe. In one embodiment, this sequence of captured images iscombined by device 114 to form one continuous fingerprint image. Theprocess for combining images is referred to as “stitching.” In oneembodiment, stitching is performed by device 114 based on navigationinformation that indicates how much the finger 102 moved betweencaptured images. In one embodiment, the navigation information isextracted by device 114 from the captured images themselves, asdescribed in further detail below. In addition to using the navigationinformation for stitching in finger recognition applications, thenavigation information may also be used to control a cursor on a displayscreen of a host device, as a rocker-switch for selecting menu items ona displayed menu, for free-hand writing, as well as other applications.

[0020]FIG. 2 is a diagram illustrating the image sensor 116 according toone embodiment of the present invention. Image sensor 116 includes astitching sensor portion 116A and a navigation sensor portion 116B.Sensor portions 116A and 116B each include a plurality of pixel circuits(pixels) 202 organized into a plurality of rows and a plurality ofcolumns. The pixels 202 in stitching sensor portion 116A capture imageinformation for generating digital images of a finger that are stitchedtogether to form a larger stitched image. The pixels 202 in navigationsensor portion 116B capture image information that is processed todetermine the amount and direction of movement of finger 102. The pixels202 that are contained in the overlapping region 204 of the two sensorportions 116A and 1161B capture image information for both stitching andfinger navigation functions.

[0021] The number of pixels 202 in image sensor 116 is defined by thedesired resolution, which is 400 dots per inch (dpi) in one form of theinvention. In order to capture a fingerprint image with enough featuresfor recognition, a certain resolution and area should be captured. Witha 12×16 millimeter (mm) area at 400 dpi resolution, approximately 64“minutiae” can be extracted. As will be understood by persons ofordinary skill in the art, “minutiae” are finger features that are usedfor recognition. Smaller and less expensive sensors that scan smallerareas (e.g., 9×12 mm) at the same (400 dpi) or lower resolution (e.g.,300 dpi) may also be used. In one embodiment, stitching sensor portion116A includes 16 rows and 144 columns of pixels 202, and navigationsensor portion 116B includes 32 rows and 32 columns of pixels 202. Inone embodiment, image sensor 116 is implemented with three separatesensors positioned adjacent to one another.

[0022]FIG. 3 is a block diagram illustrating major components of theimage sensing and processing device 114 for the fingerprint sensor 100shown in FIG. 1 according to one embodiment of the present invention.Device 114 includes image sensor 116, analog-to-digital (A/D) converter302, navigation controller 304, sensor interface 306, oscillator (Osc)308, input/output (I/O) interface 310, light source controller 312,analog-to-digital converter 314, and fingerprint controller 316. A hostsystem 318, which is communicatively coupled to device 114, is alsoshown in FIG. 3.

[0023] Oscillator 308 provides a clock signal to navigation controller304, fingerprint controller 316, and to image sensor 116 (through sensorinterface 306). Oscillator 308 is programmable by host system 318 viainput/output interface 310. Navigation controller 304 and fingerprintcontroller 316 control image sensor 116 using control signals sent tosensor interface 306.

[0024] In operation, according to one embodiment, light source 106(shown in FIG. 1) emits light that is directed onto finger 102. Lightsource 106 is controlled by signals 313 from light source controller312, which is controlled by navigation controller 304 and fingerprintcontroller 316. Reflected light from finger 102 is directed by optics110 and prism 112 (both shown in FIG. 1) onto image sensor 116. In oneform of the invention, the charge from each pixel circuit 202 isaccumulated for a predetermined exposure period, thereby creating avoltage that is related to the intensity of light incident on the pixelcircuit 202. The voltage for each pixel circuit 202 in navigation sensorportion 116B is output to analog-to-digital converter 302. The voltagefor each pixel circuit 202 in stitching sensor portion 116A is output toanalog-to-digital converter 314. Analog-to-digital converters 302 and314 convert the received voltages to digital values of a suitableresolution (e.g., one to eight bits) indicative of the level of voltage.The digital values represent a digital image or digital representationof a portion of finger 102. Navigation controller 304 and fingerprintcontroller 316 process the digital images as described below.

[0025] The digital image information output from analog-to-digitalconverter 302 to navigation controller 304 include several features(e.g., whorls of skin in the finger). Images of such spatial featuresproduce translated patterns of pixel information as finger 102 is movedrelative to surface 108A of prism 108. The number of pixel circuits 202in navigation sensor portion 116B and the frame rate at which theircontents are captured and digitized cooperate to influence how fastfinger 102 can be moved and still be tracked. Navigation controller 304tracks movement by comparing a newly captured sample frame with apreviously captured reference frame to ascertain the direction andamount of movement.

[0026] The ΔX and ΔY movement information is output from navigationcontroller 304 through input/output interface 310 to host system 318 forfinger navigation applications. For finger recognition applications, theΔX and ΔY information is provided to fingerprint controller 316 tofacilitate the stitching of images together.

[0027] Fingerprint controller 316 receives digital image informationfrom A/D converter 314. In one embodiment, fingerprint controller 316stitches the captured images together using the ΔX and ΔY informationprovided by navigation controller 304, thereby forming larger combinedor stitched images. In one form of the invention, fingerprint controller316 also performs other image processing operations on the images tomake them more suitable for fingerprint recognition. For example, in oneembodiment, fingerprint controller 316 converts captured gray scaleimages into black and white images, which essentially enhances thecontrast of the images. As will be understood by persons of ordinaryskill in the art, additional image processing operations may also beperformed. In one form of the invention, fingerprint controller 316outputs the stitched and processed images through input/output interface310 to host system 318, which performs fingerprint recognitionfunctions. In another embodiment, fingerprint controller 316 performsthe fingerprint recognition functions.

[0028] Host system 318 includes processor 320 and memory 322.Authentication application 324 and fingerprint data 326 are stored inmemory 322. Authentication application 324 is executed by processor 320,and analyzes fingerprint images output by fingerprint controller 316 toauthenticate or verify the user of host system 318. In one embodiment,host system 318 is a portable electronic device, such as a cellulartelephone, PDA, or laptop computer.

[0029] In one embodiment, fingerprint data 326 includes a unique set ofdata for each authorized user of host system 318. The fingerprint data326 is obtained during an enrollment phase, and is stored in memory 326.In one form of the invention, during the enrollment phase, an authorizeduser is requested by host system 318 to swipe his finger in a downwarddirection on swipe surface 108A of swipe sensor 100, and then to swipehis finger upward on swipe surface 108A. Fingerprint controller 316generates corresponding swipe down and swipe up stitched images of theuser's finger, which are output to host system 318. Host system 318analyzes the swipe down and swipe up stitched images, and extractsminutiae from the two images. In one embodiment, host system 318 alsodetermines a minutiae delta value from the swipe down and swipe upstitched images. Minutiae delta values are described in further detailbelow with reference to FIGS. 4A-4C. The extracted minutia and theassociated minutiae delta value uniquely identify the user, and arestored as a set of fingerprint data 326 in memory 322.

[0030] After the enrollment phase, host system 318 can authenticate orverify whether a potential user of system 318 is an authorized user. Inone form of the invention;, a potential user is requested by host system318 to swipe his finger in a downward direction on swipe surface 108A ofswipe sensor 100, and then to swipe his finger upward on swipe surface108A. Fingerprint controller 316 generates corresponding swipe down andswipe up stitched images of the potential user's finger, which areoutput to host system 318. Host system 318 analyzes the swipe down andswipe up stitched images, and extracts minutiae from the two images. Inone embodiment, host system 318 also determines a minutiae delta valuefrom the swipe down and swipe up stitched images. In one form of theinvention, under the control of authentication application 324,processor 320 compares the extracted minutiae and the minutiae deltavalue for the potential user with the previously stored fingerprint data326, and determines whether the fingerprint data 326 includes a matchingset of data. In one form of the invention, the potential user isprevented from operating host system 318 until the user's fingerprinthas been properly authenticated by authentication application 324.

[0031] It will be understood by a person of ordinary skill in the artthat functions performed by image sensing and processing device 114 andhost system 318 may be implemented in hardware, software, firmware, orany combination thereof. The implementation may be via a microprocessor,programmable logic device, or state machine. Components of the presentinvention may reside in software on one or more computer-readablemediums. The term computer-readable medium as used herein is defined toinclude any kind of memory, volatile or non-volatile, such as floppydisks, hard disks, CD-ROMs, flash memory, read-only memory (ROM), andrandom access memory.

[0032] FIGS. 4A-4C are diagrams illustrating simplified representationsof fingerprint images for the same finger. FIG. 4A is a simplifieddiagram illustrating an example fingerprint image 400 captured by anoptical area sensor. FIG. 4B is a simplified diagram illustrating astitched fingerprint image 410 generated from images captured during aswipe-down motion according to one embodiment of the present invention.FIG. 4C is a simplified diagram illustrating a stitched fingerprintimage 420 generated from images captured during a swipe-up motionaccording to one embodiment of the present invention. Images 400, 410,and 420 each include the same minutiae 402A-402C (collectively referredto as minutiae 402). However, the distances or spacing between theminutiae and the pitch of the ridges are not the same for images 400,410, and 420, for reasons that are explained in further detail below.The distances between minutiae 402A and 402B in images 400, 410, and420, are represented by the reference numbers 404, 412, and 422,respectively.

[0033] The distance 412 between minutiae 402A and 402B in the swipe-downimage 410 (FIG. 4B) is the smallest of the three distances 404, 412, and422. Similarly, the spacing between ridges (i.e., the ridge pitch) forthe swipe-down image 410 is the smallest of the three images 400, 410,and 420. The distance 422 between the same two minutiae 402A and 402B inthe swipe-up image 420 (FIG. 4C) is the largest of the three distances404, 412, and 422. And the spacing between ridges for the swipe-up image420 is the largest of the three images 400, 410, and 420.

[0034] More generally, for the swipe-down image 410, the ridge pitch isnarrower and the minutiae 402 are closer together than for images 400and 420. For the swipe-up image 420, the ridge pitch is wider and theminutiae 402 are stretched farther apart than for images 400 and 410.The difference in ridge pitch and minutiae spacing between the images400, 410, and 420, is a result of the deformation of the finger when itis swiped up or swiped down on sensor 100. A real human's finger, unlikea photocopy of a finger, is soft. So the finger deforms while swipingdown and swiping up on the sensor 100. The deformation during swipingdown of the finger is different than the deformation during swiping upof the finger. Correspondingly, the stitched image of a swipe downmovement (e.g., image 410) is different than the stitched image of aswipe up movement (e.g., image 420). More specifically, during aswipe-down movement (i.e., toward the user's wrist), the fingertip 104is pushed against the tip of the fingernail, thereby narrowing the ridgepitch and moving the minutiae 402 close together. During a swipe-upmovement (i.e., away from the user's wrist), the fingertip 104 is pushedby the user away from the wrist and pulled by friction toward the wrist,thereby widening the ridge pitch and moving the minutia 402 fartherapart.

[0035] Assuming that distance 412 is 2 millimeters (mm) and thatdistance 422 is 4 mm, the difference between the two distances 412 and422 would be 2 mm. This difference is referred to as the minutiae delta.If distance 412 is 2 mm, and distance 422 is 4 mm, the distance 404 forimage 400 would be somewhere between 2 mm and 4 mm (e.g., about 3 mm).

[0036] In contrast to a real human finger, a photocopy does not deformlike an actual human finger when it is swiped up or swiped down on aswipe sensor, such as sensor 100. The stitched images for swipe up andswipe down movements of the photocopy will appear the same. Thus, for aphotocopy, the minutiae delta will be zero. In one embodiment, theeffects of finger deformation are taken into account by host system 318to distinguish between a real human finger and a fake finger. In oneform of the invention, host system 318 distinguishes between a realhuman finger and a fake finger based on minutiae delta values, asdescribed in further detail below with reference to FIGS. 5 and 6.

[0037]FIG. 5 is a flow diagram illustrating a fingerprint enrollmentmethod 500 performed by sensor 100 and host system 318 according to oneembodiment of the present invention. In step 502, a user swipes hisfinger 102 down on prism 108, and a first set of images of the finger102 are captured by image sensor 116. In step 504, a user swipes hisfinger 102 up on prism 108, and a second set of images of the finger 102are captured by image sensor 116. In step 506, the first set of imagesis stitched together by device 114 to form a swipe down stitched image,and the second set of images are stitched together to form a swipe upstitched image. In step 508, the two stitched images are output fromdevice 114 to host system 318.

[0038] In step 510, host system 318 extracts minutiae from the swipedown stitched image. In step 512, host system 318 determines if thenumber of minutiae extracted from the swipe down stitched image isgreater than a predetermined threshold number. If the number of minutiaeextracted from the swipe down stitched image is not greater than thepredetermined threshold number, the method returns to step 502 to repeatthe image capture process. If the number of minutiae extracted from theswipe down stitched image is greater than the predetermined thresholdnumber, the method moves on to step 514. In step 514, the extractedminutiae (represented herein as ED 1) for the swipe down stitched imageare stored in memory 322 of host system 318.

[0039] In step 516, host system 318 extracts minutiae from the swipe upstitched image. In step 518, host system 318 determines if the number ofminutiae extracted from the swipe up stitched image is greater than apredetermined threshold number. If the number of minutiae extracted fromthe swipe up stitched image is not greater than the predeterminedthreshold number, the method returns to step 502 to repeat the imagecapture process. If the number of minutiae extracted from the swipe upstitched image is greater than the predetermined threshold number, themethod moves on to step 520. In step 520, the extracted minutiae(represented herein as ED2) for the swipe up stitched image are storedin memory 322 of host system 318.

[0040] In step 522, host system 318 calculates a minutiae delta valuefor the swipe down and swipe up stitched images. In one embodiment, theminutiae delta value is calculated by comparing the swipe down and swipeup stitched images, and determining the difference in the distancebetween two minutiae in the swipe down stitched image and the distancebetween the same two minutiae in the swipe up stitched image. In anotherembodiment, other techniques may be used to represent the differencebetween the swipe down and swipe up images caused by deformation of thefinger. In step 524, the calculated minutiae delta value (representedherein as EDD) is stored in memory 322 of host system 318.

[0041] After the enrollment process, host system 318 can authenticate orverify new fingerprints based on the fingerprint data 326 obtainedduring enrollment. FIG. 6 is a flow diagram illustrating a fingerprintauthentication method 600 performed by sensor 100 and host system 318according to one embodiment of the present invention. In step 602, auser swipes his finger 102 down on prism 108, and a first set of imagesof the finger 102 are captured by image sensor 116. In step 604, a userswipes his finger 102 up on prism 108, and a second set of images of thefinger 102 are captured by image sensor 116. In step 606, the first setof images is stitched together by device 114 to form a swipe downstitched image, and the second set of images are stitched together toform a swipe up stitched image. In step 608, the two stitched images areoutput from device 114 to host system 318.

[0042] In step 610, host system 318 extracts minutiae (representedherein as AD1) from the swipe down stitched image. In step 612, hostsystem 318 extracts minutiae (represented herein as AD2) from the swipeup stitched image. In step 614, host system 318 calculates a minutiaedelta value (represented herein as ADD) for the swipe down and swipe upstitched images. In one embodiment, the minutiae delta value iscalculated by comparing the swipe down and swipe up stitched images, anddetermining the difference in the distance between two minutiae in theswipe down stitched image and the distance between the same two minutiaein the swipe up stitched image.

[0043] In step 616, host system 318 determines if the data AD1, AD2, andADD, matches fingerprint data 326 obtained during the enrollmentprocess. In one embodiment, if host system 318 determines that AD1matches ED1, AD2 matches ED2, and ADD matches EDD, host system 318accepts the person as an authorized user, as indicated in step 618. IfAD1 does not match ED1, or AD2 does not match ED2, or ADD does not matchEDD, host system 318 rejects the person as not being an authorized user,as indicated in step 620, and the method returns to step 602.

[0044] One form of the present invention provides a system with anoptical fingerprint swipe sensor with better recognition accuracy thanprior fingerprint sensors. In one form of the invention, enhancedrecognition accuracy is provided by performing two scans (e.g., a swipedown scan and a swipe up scan), which allows additional minutiae to beextracted. In one embodiment, the system distinguishes between a realhuman finger and a fake finger.

[0045] Although specific embodiments have been illustrated and describedherein for purposes of description of the preferred embodiment, it willbe appreciated by those of ordinary skill in the art that a wide varietyof alternate and/or equivalent implementations may be substituted forthe specific embodiments shown and described without departing from thescope of the present invention. Those with skill in the mechanical,electro-mechanical, electrical, and computer arts will readilyappreciate that the present invention may be implemented in a very widevariety of embodiments. This application is intended to cover anyadaptations or variations of the preferred embodiments discussed herein.Therefore, it is manifestly intended that this invention be limited onlyby the claims and the equivalents thereof.

What is claimed is:
 1. A fingerprint verification device comprising: afingerprint sensor including an imaging surface, the fingerprint sensorconfigured to capture images of a user's finger when the finger isswiped against the imaging surface; a memory for storing fingerprintdata; and a controller for comparing minutiae in the captured imageswith the stored fingerprint data to verify the user's identity, thecontroller configured to determine whether the finger is a real humanfinger based on the spacing of minutiae in the captured images.
 2. Thefingerprint verification device of claim 1, wherein the fingerprintsensor is configured to capture a first set of images of the finger whenthe finger is swiped in a first direction on the imaging surface, andcapture a second set of images of the finger when the finger is swipedin a second direction on the imaging surface.
 3. The fingerprintverification device of claim 2, wherein the fingerprint sensor isconfigured to generate a first stitched image based on the first set ofimages, and a second stitched image based on the second set of images,and wherein the controller is configured to compare minutiae in thefirst and the second stitched images with the stored fingerprint data toverify the user's identity.
 4. The fingerprint verification device ofclaim 3, wherein the controller is configured to compare the spacingbetween minutiae in the first stitched image with the spacing betweenminutiae in the second stitched image.
 5. The fingerprint verificationdevice of claim 4, wherein the controller is configured to determinewhether the finger is a real human finger based on the comparison of thespacing between minutiae in the first and the second stitched images,and the stored fingerprint data.
 6. The fingerprint verification deviceof claim 3, wherein the controller is configured to calculate a deltavalue representing a difference between the spacing of minutiae in thefirst stitched image and the spacing of minutiae in the second stitchedimage.
 7. The fingerprint verification device of claim 6, wherein thecontroller is configured to compare the delta value to the storedfingerprint data to determine whether the finger is a real human finger.8. The fingerprint verification device of claim 7, wherein thecontroller is configured to calculate a first value representing thespacing between a first minutia and a second minutia in the firststitched image, calculate a second value representing the spacingbetween the first minutia and the second minutia in the second stitchedimage, and wherein the delta value represents the difference between thefirst value and the second value.
 9. The fingerprint verification deviceof claim 1, wherein the fingerprint sensor is an optical swipe sensor.10. A method of recognizing a fingerprint, the method comprising:providing a fingerprint sensor; capturing a first set of images of afinger when the finger is swiped in a first direction on the fingerprintsensor; stitching the first set of images together to form a firststitched image; capturing a second set of images of the finger when thefinger is swiped in a second direction on the fingerprint sensor;stitching the second set of images together to form a second stitchedimage; comparing minutiae in the first and the second stitched images tostored fingerprint data; and determining whether the finger isrecognizable based on the comparison.
 11. The method of claim 10, andfurther comprising: comparing the distance between features in the firststitched image with the distance between features in the second stitchedimage.
 12. The method of claim 11, and further comprising: determiningwhether the finger is a real human finger based on the comparison of thedistance between features, and the stored fingerprint data.
 13. Themethod of claim 10, and further comprising: calculating a delta valuerepresenting a difference between the distance between features in thefirst stitched image and the distance between features in the secondstitched image.
 14. The method of claim 13, and further comprising:comparing the delta value to the stored fingerprint data to determinewhether the finger is a real human finger.
 15. The method of claim 14,and further comprising: calculating a first value representing thedistance between a first minutia and a second minutia in the firststitched image; calculating a second value representing the distancebetween the first minutia and the second minutia in the second stitchedimage; and wherein the delta value represents the difference between thefirst value and the second value.
 16. The method of claim 10, andfurther comprising: generating movement information with the fingerprintsensor, the movement information indicative of movement of the finger onthe fingerprint sensor; and wherein the first and the second stitchedimages are formed based on the movement information.
 17. An electronicdevice comprising: an optical swipe sensor including an imaging surface,the optical swipe sensor configured to capture a first set of images ofa user's finger when the finger is swiped in a first direction on theimaging surface, and capture a second set of images of the finger whenthe finger is swiped in a second direction on the imaging surface,wherein the second direction is substantially opposite to the firstdirection; a memory for storing fingerprint data; and a processor forextracting minutiae from the first and the second set of images, theprocessor configured to compare the extracted minutiae with the storedfingerprint data to verify the user's identity, the processor configuredto distinguish between a real human finger and a fake finger based onthe spacing of features appearing in the captured images.
 18. Theelectronic device of claim 17, wherein the optical swipe sensor isconfigured to generate a first stitched image based on the first set ofimages, and a second stitched image based on the second set of images.19. The electronic device of claim 18, wherein the processor isconfigured to compare the spacing between minutiae in the first stitchedimage with the spacing between minutiae in the second stitched image.20. The electronic device of claim 19, wherein the processor isconfigured to distinguish between a real human finger and a fake fingerbased on the comparison of the spacing between minutiae in the first andthe second stitched images, and the stored fingerprint data.